In the field of optical communication, communication systems that combine a synchronous detection scheme and signal processing for dramatically improving spectral efficiency are attracting attention. This type of communication system can further improve reception sensitivity than systems constructed in accordance with direction detection. Further, such a communication system receives a transmission signal as a digital signal, and thus it can perform signal position detection, frequency offset compensation, clock offset compensation, and compensation for waveform distortion due to linear effects, such as chromatic dispersion compensation and polarization-mode dispersion (PMD) compensation by signal processing of the received digital signal. In addition, it is known that such a communication system has strong tolerance even to signal quality deterioration due to non-linear effects by performing digital compensation. Therefore, introduction of such a communication system is being examined as a next generation optical communication technology.
A digital coherent scheme as described in Non-Patent Documents 1 and 2 adopts a method for compensating for quasi-static chromatic dispersion by means of a digital filter having a fixed number of taps (e.g., the number of taps is 2048 for a dispersion of 20000 ps/nm and a signal of 28 Gbaud) and compensating for fluctuating polarization-mode dispersion by means of an adaptive filter having a small number of taps (e.g., about 10 to 12 taps for a polarization-mode dispersion of 50 ps) using a blind algorithm. Also, as described in Non-Patent Document 3, polarization-division multiplexing transmission is attracting attention with an increase of a transfer rate.
It is to be noted that Non-Patent Document 4 describes establishment of synchronous in wireless communication. Moreover, Non-Patent Document 5 describes cross-phase modulation, which is a non-linear optical effect caused by an adjacent wavelength, in wavelength-division multiplexing transmission.
As described in Non-Patent Document 4, in the IEEE 802.11a standard, which is a wireless local area network (LAN) standard in wireless communication, it is possible to estimate frequency offset and clock offset by means of a frame configuration in which a short preamble signal and/or a long preamble signal is inserted as a training signal into a head of a transmission signal. Then, it is possible to establish synchronization by compensating for these offsets based on an estimated result.
On the other hand, because there is a problem unique to an optical signal, such as chromatic dispersion, in optical communication, it is difficult to correctly detect a received bit due to the chromatic dispersion and thus it is difficult to detect the above-described short preamble signal and long preamble signal at a receiving end. With respect to this point, it is possible to detect a training pattern by generating a specific frequency band signal having signal components with a smaller frequency spreading relative to a spectrum of a signal sequence to be transmitted at a plurality of specific frequencies, inserting the generated specific frequency band signal into the signal sequence to be transmitted, and providing a circuit having high detection sensitivity for the specific frequency band signal at the receiving end (Non-Patent Document 1).
Also, by individually detecting a plurality of specific frequency band signals at the receiving end, it is possible to estimate a chromatic dispersion amount from their arrival time differences and their central frequency differences. Further, by estimating the central frequency of the specific frequency band signal, it is possible to estimate a frequency offset between a transmitting-end laser and a receiving-end local oscillator laser. As a result, it is possible to detect the chromatic dispersion and the frequency offset for even a signal of about 100 Gbp using the above-described means.